Device and method for cascading filters of different materials

ABSTRACT

Some embodiments of the invention provide a filter having at least one first filter, each first filter being a band-reject type filter having a first set of filter parameters that are a function of a first material used to fabricate the at least one first filter, and at least one second filter, each second filter having a second set of filter parameters that are a function of a second material used to fabricate the at least one second filter, each second filter being one of a band-reject type filter and a band pass type filter. The at least one first filter and the at least one second filter are then cascaded together to form the filter. The first material and the second material are different materials. The cascaded filter has a new third set of filter parameters that are a function of both the first material and the second material. Other embodiments of the invention include a method for fabricating the filter and a method of filtering using such a cascaded filter.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/424,068 filed on Apr. 15, 2009, which is incorporated herein byreference in its entirety, and claims the benefit thereof.

FIELD OF THE INVENTION

The invention relates to cascading multiple filters.

BACKGROUND OF THE INVENTION

There is a strong need in the telecommunications market, particularly inthe area of 4G wireless communication systems, as well as in existingwireless systems, for miniature type filters with improved performancefrom current levels. As 4G systems target a very high speed datatransfer, they need much wider bandwidth than existing systems such asGSM, CDMA and UMTS. On the other hand, limited frequency resources in 4Gsystems require wireless carrier companies to set guard-bands as narrowas possible to enable maximum user capacity. Combining these two issuesmeans that the 4G wireless systems require miniature RF filters fortheir wireless terminal devices that not only have a wide pass band orreject-band, but also have steep transition bands.

Due to their miniature size and low cost, acoustic materials-based RFfilters such as surface acoustic wave (SAW), thin film bulk acousticresonator (FBAR) and/or bulk acoustic wave (BAW) filters are widely usedin compact and portable type terminal devices of various wirelesssystems. However, the current level of filter performance of thesefilters is still far from 4G wireless system filter requirements.

Some non-acoustic microwave technology type filters, such as metal-typecavity filters or dielectric filters can be designed to meet filterperformance requirements for these applications, but these types ofdesigns have an ultra-high cost and result in physically large filters.As a result, metal-type cavity filters and dielectric filters areundesirable, particularly for applications in wireless terminals, forwhich size and weight are of considerable importance.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a filtercomprising: at least one first filter, each first filter being aband-reject type filter having a first set of filter parameters that area function of a first material used to fabricate the respective firstfilter; at least one second filter, each second filter having a secondset of filter parameters that are a function of a second material usedto fabricate the respective second filter, each second filter being oneof: a band-reject type filter; and a band pass type filter; wherein atleast one of the at least one first filter and at least one of the atleast one second filter are cascaded together; wherein the firstmaterial and the second material are different materials; and whereinthe filter has a third set of filter parameters that are a function ofboth the first material and the second material.

In some embodiments, each first filter is a narrow band band-reject typefilter with a filter response having at least one rejection band, eachrejection band having steep transition bands relative to transitionbands of each second filter.

In some embodiments, the first material has a smaller magnitudetemperature coefficient than the second material such that each firstfilter has less temperature dependent frequency drift than each secondfilter.

In some embodiments, each second filter is one of: a wide band band passtype filter; and a wide band band-reject type filter.

In some embodiments, the second material has a higher electro-mechanicalcoupling coefficient than the first material.

In some embodiments, each first filter has one of: a first rejectionband being arranged at a low side edge of a passband of one of the atleast one second filter; a first rejection band being arranged at a lowside edge of a rejection band of one of the at least one second filter;a first rejection band being arranged at a high side edge of a passbandof one of the at least one second filter; a first rejection band beingarranged at a high side edge of a rejection band of one of the at leastone second filter; two rejection bands, a first rejection band of thetwo rejection bands being arranged at a low side edge of a passband ofone of the at least one second filter and a second rejection band of thetwo rejection bands being arranged at a high side edge of the passbandof one of the at least one second filter; and two rejection bands, afirst rejection band of the two rejection bands being arranged at a lowside edge of a rejection band of one of the at least one second filterand a second rejection band of the two rejection bands being arranged ata high side edge of the rejection band of one of the at least one secondfilter.

In some embodiments, each first filter is fabricated using any one of:surface acoustic wave (SAW) technology; thin film bulk acousticresonator (FBAR) technology; and bulk acoustic wave (BAW) filtertechnology; and each second filter is fabricated using any one of: SAWtechnology; FBAR technology; and BAW filter technology.

In some embodiments, the first material comprises at least one of:Quartz; Langasite; SiO₂/ZnO/Diamond; SiO₂/AlN/Diamond; Li₂B₄O₇;AlN/Li₂B₄O₇; LiTaO₃; LiNbO₃; SiO₂/LiTaO₃; SiO₂/LiNbO₃; AlN; andcombinations thereof.

In some embodiments, the second material comprises at least one of:Quartz; Langasite; SiO₂/ZnO/Diamond; SiO₂/AlN/Diamond; Li₂B₄O₇;AlN/Li₂B₄O₇; LiTaO₃; LiNbO₃; SiO₂/LiTaO₃; SiO₂/LiNbO₃; ZnO; AlN; andcombinations thereof.

In some embodiments, a first filter of the at least one first filter anda second filter of the at least one second filter are cascaded togetherin a package using at least one of: a link directly electricallyconnecting the first filter and the second filter; and a shared point ofconnection within the package to which the first filter and the secondfilter are electrically connected.

In some embodiments, the filter further comprises at least one of: acircuit matching element for matching at least one of an input to thefilter and an output from the filter; a circuit matching element formatching a first filter of the at least one first filter; a circuitmatching element for matching a second filter of the at least one secondfilter; and a circuit matching element for matching a point in thefilter at which a first filter of the at least one first filter and asecond filter of the at least one second filter are cascaded together.

According to a second aspect of the invention there is provided a methodfor fabricating a filter comprising: cascading at least one firstfilter, each first filter being a band-reject type filter having a firstset of filter parameters that are a function of a first material used tofabricate the respective first filter, together with at least one secondfilter, each second filter having a second set of filter parameters thatare a function of a second material used to fabricate the respectivesecond filter, each second filter being one of: a band-reject typefilter; and a band pass type filter; wherein the first material and thesecond material are different materials; and wherein the filter has athird set of filter parameters that are a function of both the firstmaterial and the second material.

In some embodiments, cascading at least one first filter and at leastone second filter comprises: cascading a first filter of the at leastone first filter, the first filter being a narrow band band-reject typefilter with a filter response having at least one rejection band, eachrejection band having steep transition bands relative to transitionbands of each of the at least one second filter, with a second filter ofthe at least one second filter.

In some embodiments, the first material has a smaller magnitudetemperature coefficient than the second material, such that each firstfilter of the at least one first filter has less temperature dependentfrequency drift than each second filter of the at least one secondfilter.

In some embodiments, cascading at least one first filter and at leastone second filter comprises: cascading a first filter of the at leastone first filter with a second filter of the at least one second filter,the second filter being one of: a wide band band pass type filter; and awide band band-reject type filter.

In some embodiments, the second material has a higher electro-mechanicalcoupling coefficient than the first material.

In some embodiments, cascading the at least one first filter and the atleast one second filter comprises: cascading a first filter of the atleast one first filter, wherein the first filter is fabricated using anyone of: surface acoustic wave (SAW) technology; thin film bulk acousticresonator (FBAR) technology; and bulk acoustic wave (BAW) filtertechnology; with a second filter of the at least one second filter,wherein the second filter is fabricated using any one of: SAWtechnology; FBAR technology; and BAW filter technology.

In some embodiments, each first filter has one of: a first rejectionband being arranged at a low side edge of a passband of one of the atleast one second filter; a first rejection band being arranged at a lowside edge of a rejection band of one of the at least one second filter;a first rejection band being arranged at a high side edge of a passbandof one of the at least one second filter; a first rejection band beingarranged at a high side edge of a rejection band of one of the at leastone second filter; two rejection bands, a first rejection band of thetwo rejection bands being arranged at a low side edge of a passband ofone of the at least one second filter and a second rejection band of thetwo rejection bands being arranged at a high side edge of the passbandof one of the at least one second filter; and two rejection bands, afirst rejection band of the two rejection bands being arranged at a lowside edge of a rejection band of one of the at least one second filterand a second rejection band of the two rejection bands being arranged ata high side edge of one of the rejection band of the at least one secondfilter.

In some embodiments, cascading at least one first filter and at leastone second filter comprises: cascading a first filter of the at leastone first filter with a second filter of the at least one second filter,wherein the first filter is fabricated using the first material, whichcomprises at least one of: Quartz; Langasite; SiO₂/ZnO/Diamond;SiO₂/AlN/Diamond; Li₂B₄O₇; AlN/Li₂B₄O₇; LiTaO₃; LiNbO₃; SiO₂/LiTaO₃;SiO₂/LiNbO₃; AlN; and combinations thereof.

In some embodiments, cascading at least one first filter and at leastone second filter comprises: cascading a first filter of the at leastone first filter with a second filter of the at least one second filter,wherein the second filter is fabricated using the second material, whichcomprises at least one of: Quartz; Langasite; SiO₂/ZnO/Diamond;SiO₂/AlN/Diamond; Li₂B₄O₇; AlN/Li₂B₄O₇; LiTaO₃; LiNbO₃; SiO₂/LiTaO₃;SiO₂/LiNbO₃; ZnO; AlN; and combinations thereof.

In some embodiments, cascading at least one first filter and at leastone second filter comprises: cascading a first filter of the at leastone first filter with a second filter of the at least one second filtertogether in a package using at least one of: a link directlyelectrically connecting the first filter and the second filter; and ashared point of connection to which the first filter and the secondfilter are electrically connected.

In some embodiments, cascading the at least one first filter and the atleast one second filter comprises: circuit matching at least one of aninput to the device and an output from the filter; circuit matching afirst filter of the at least one first filter; circuit matching a secondfilter of the at least one second filter; and circuit matching a pointin the filter at which a first filter of the at least one first filterand a second filter of the at least one second filter are cascadedtogether.

According to a third aspect of the invention, there is provided methodfor filtering a signal comprising: providing a signal to an input of afirst filter, the first filter being a band-reject type filter having afirst set of filter parameters that are a function of a first materialused to fabricate the first filter; filtering the signal using the firstfilter thereby producing an output of the first filter; providing theoutput of the first filter to a second filter, the second filter havinga second set of filter parameters that are a function of a secondmaterial used to fabricate the second filter, the second filter beingone of a band-reject type filter and a band pass type filter; filteringthe output of the first filter using the second filter thereby producingan output of the second filter; wherein the first material and thesecond material are different materials; and wherein the combination ofthe first filter and the second filter has a third set of filterparameters that are a function of both the first material and the secondmaterial.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theattached drawings in which:

FIG. 1A is a graphical plot of a pair of wide band pass band filterresponses, one response at a high temperature and one response at a lowtemperature;

FIG. 1B is a graphical plot of a pair of wide band rejection band filterresponses, one response at a high temperature and one response at a lowtemperature;

FIG. 2A is a graphical plot of a pair of narrow band pass band filterresponses, one response at a high temperature and one response at a lowtemperature;

FIG. 2B is a graphical plot of a pair of narrow band band-reject filterresponses, one response at a high temperature and one response at a lowtemperature;

FIG. 3 is a block diagram of two filters cascaded in series inaccordance with some embodiments of the invention;

FIGS. 4A to 4E are graphical plots of filter responses of individualfilters and a pass band filter response for a filter cascading theindividual filters in accordance with some embodiments of the invention;

FIGS. 5A to 5C are graphical plots of filter responses of individualfilters and a pass band filter response for a filter cascading theindividual filters in accordance with some embodiments of the invention;and

FIGS. 6A to 6C are graphical plots of filter responses of individualfilters and a band-reject filter response for a filter cascading theindividual filters in accordance with some embodiments of the invention;

FIGS. 7A to 7C are graphical plots of filter responses of individualfilters and a band-reject filter response for a filter cascading theindividual filters in accordance with some embodiments of the invention;

FIG. 8 is a block diagram of a cascaded filter and matching networks inaccordance with some embodiments of the invention;

FIG. 9A is a schematic diagram of a direct wire bond connection betweentwo cascaded filters in a package according to an embodiment of theinvention;

FIG. 9B is a schematic diagram of a shared circuit pad electricalconnection between two cascaded filters in a package according to anembodiment of the invention;

FIG. 9C is a schematic diagram of flip-chip implementation for twocascaded filters according to an embodiment of the invention;

FIG. 10 is a flow chart of a method for implementing some embodiments ofthe invention; and

FIG. 11 is a flow chart of another method for implementing someembodiments of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Due to the desire for miniature sizing and low cost, surface acousticwave (SAW), thin film bulk acoustic resonator (FBAR) and/or bulkacoustic wave (BAW) technology filters have became much utilizedcomponents in compact and portable type terminal devices for variousmodern wireless communication systems. Band pass type and band-rejecttype filters can be designed using SAW, FBAR and BAW technologies.However, current SAW, BAW and FBAR filter design technologies can notprovide a filter solution having further improved filter performancesuch as steeper transition band and higher power handling capability.After over 30 years of filter technology development for SAW, 15 yearsfor FBAR and 10 years for BAW, it can be said that substantially closeto maximum filter performance for these types of devices has beenreached. Therefore, no large scale performance improvements are likelyto occur for single substrate filters based on existing SAW, FBAR andBAW filter design technologies, unless new materials are realized.

Materials for fabricating filters with high electro-mechanical couplingcoefficients are suitable for implementing band-reject or band passfilters with respective wide transition bands and a wide rejection bandor wide pass band. However, usually the materials have poor temperaturestability due to a large magnitude temperature coefficient, whichresults in a frequency response in devices made from the material havinga large temperature dependent frequency response drift. Current wideband filters designed using SAW, FBAR and BAW technologies have twoparticularly troubling drawbacks: transition band bandwidths that arenot steep enough and large temperature dependent frequency responsedrift. FIG. 1A illustrates an example of a computer simulation of a bandpass filter response for a single substrate material filter that couldbe fabricated by any one of SAW, FBAR, or BAW technologies. Thefrequency range indicated along the x-axis is from 1.30 GHz to 1.55 GHz.The attenuation range on the y-axis ranges from 10 dB to −100 dB. Afirst filter response 10 in FIG. 1A is a frequency response of thefilter operating at a temperature of approximately 85° C. The 3 dBbandwidth of the first filter response 10 is in the range ofapproximately 0.080 GHz, from 1.370 GHz to 1.450 GHz. A second filterresponse 12 in FIG. 1A is a frequency response of the same filteroperating at a temperature of approximately −40° C. The 3 dB bandwidthof the second filter response 12 is in the same range of approximately0.080 GHz, from 1.380 GHz to 1.460 GHz. For each of the frequencyresponses 10,12 it can be seen that the transition band on either sideof the pass band is rather large, for example, for the filter response10 in FIG. 1A a 20 dB down transition bandwidth is approximately 0.010GHz, from 1.360 GHz to 1.370 GHz on the lower side of the pass band andapproximately 0.010 GHz, from 1.460 GHz to 1.470 GHz on the higher sideof the pass band. The temperature dependent frequency response drift,the change in frequency at a similar attenuation value, between the hightemperature and the low temperature filter responses 10 and 12 isapproximately equal to 0.010 GHz.

FIG. 1B illustrates band-reject filter responses for a high temperature14 and a low temperature 16 showing a similar frequency response driftover temperature and a similarly wide transition band that isillustrated for the filter responses of FIG. 1A.

There is almost no room in existing SAW, FBAR and BAW designtechnologies for further improvement to the narrowing of the transitionband bandwidth. Currently, the maximum achievable transition bandsteepness in such filters is limited by the Q factor inherent in thematerials used in the filters. A high Q factor enables the steeptransition band filter characteristic. However, the materials with ahigh electro-mechanical coupling coefficient, which is a materialproperty that enables the wide bandwidth filter characteristic, ingeneral, have a lower Q factor in comparison to the materials with a lowelectro-mechanical coupling coefficient. High electro-mechanicalcoupling coefficient materials also typically have poorer temperaturestability. Therefore, the transition band bandwidth of wide band typefilters, such as those shown in FIG. 1A and 1B, is relatively wide andthe filter response varies more significantly over the operatingtemperature range.

Materials for fabricating filters with low temperature coefficients aresuitable for implementing band-reject or band pass filters having verylow temperature dependent frequency drift. Materials with a lowtemperature coefficient typically also have a low coupling coefficient.A low coupling coefficient results in a narrow band filter.

Quartz is one of the most temperature-stable substrates in crystaldevice technology and has a very high Q, but its coupling coefficient isquite small, for example 0.11% in some SAW implementations. As such,Quartz is very good for designing narrow band type filters having a verysharp transition band.

FIG. 2A illustrates an example of a computer simulation of a band passfilter response for a single substrate material filter that may havebeen fabricated by any one of SAW, FBAR, or BAW technologies. Thefrequency and attenuation ranges are similar to FIG. 1A discussed above.A first filter response 20 of the filter in FIG. 2A is a frequencyresponse operating at a temperature of approximately 85° C. The 3 dBbandwidth of the first filter response 20 is in the range ofapproximately 0.008 GHz, from 1.442 GHz to 1.450 GHz. A second filterresponse 22 in FIG. 2A is a frequency response of the same filteroperating at a temperature of approximately −40° C. The 3 dB bandwidthof the second filter response 22 is in the range of approximately 0.008GHz, from 1.443 GHz to 1.451 GHz. The 3 dB bandwidth is approximately1/10 of the 3 dB bandwidth shown in FIG. 1A for the device fabricatedwith a higher coupling coefficient material. For each of the frequencyresponses 20,22 it can be seen that the transition band on either sideof the pass band is rather small, especially in relation to transitionband of the wide band filter responses of FIG. 1A. For example, a 20 dBdown transition bandwidth of the filter response of FIG. 2A isapproximately 0.001 GHz, from 1.442 GHz to 1.443 GHz on the lower sideof the pass band and 1.450 GHz to 1.451 GHz on the higher side of thepass band. The transition band bandwidth is approximately 1/10 of thetransition band bandwidth shown in FIG. 1A for the device fabricatedwith a higher coupling coefficient material. The temperature dependentfrequency response drift, the change in frequency at a similarattenuation value, between filter responses 20 and 22 is approximatelyequal to 0.002 GHz. This is approximately ⅕ of the temperature dependentfrequency response drift shown in FIG. 1A for the device fabricated witha higher coupling coefficient material. FIG. 2B illustrates aband-reject filter response showing a similar frequency response driftover temperature and a similarly narrow transition band.

The filter response parameters associated with the examples of FIGS. 1A,1B, 2A and 2B are merely exemplary in nature. The parameters involved indesigning filters for any given application are implementation specific.

One way of improving the drawback of large temperature dependentfrequency drift in a SAW filter fabricated with the high couplingcoefficient material is to deposit a thin film of SiO₂ over the highcoupling coefficient material. SiO₂ has a temperature coefficientopposite to that of the high coupling coefficient materials used for SAWdesigns. Therefore, the thin film of SiO₂ compensates for the hightemperature coefficient of the high coupling coefficient material.However, this process adversely affects achievable filter performancebecause the SiO₂ film reduces the effective coupling coefficient of thecombined SiO₂ coated high coupling coefficient material, and as aresult, the achievable maximum bandwidth of the SAW filter is decreased.In addition, due to a mass loading effect caused by the SiO₂ film, phasevelocity of the SAW on the combined substrates is reduced, whichcorresponds to a reduction in the width of electrode fingers in thephysical implementation of the SAW devices. This is undesirable for highfrequency (>2 GHz) SAW filter designs.

In some embodiments of the invention a filter solution is provided thatis appropriate for a guard-band-reduced special type 1900 MHz CDMAand/or 1.5 GHz to 2.5 GHz WiMAX wireless systems. These wireless systemswould benefit from a high performance filter that has low insertionloss, high power handling capability and a very narrow transition band.Furthermore, filters for such applications are desired to be low costand very compact such that they can be used in wireless terminals, suchas cellular telephones, wireless enabled PDAs computer, etc. While someguard-band-reduced special type 1900 MHz CDMA and/or 1.5 GHz to 2.5 GHzWiMAX wireless systems may benefit from embodiments of a filter solutionas described herein, it is to be understood that embodiments of theinvention may be applicable for other communication standards operatingin other frequency bands.

In general, for SAW, FBAR and BAW fabricated filters, wide band bandpass type filters or wide band band-reject type filters (3% or above)with a low insertion loss requirement (<3 dB) are designed using highcoupling coefficient materials (K²>2%). The higher the couplingcoefficient of the material used, the wider the maximum bandwidth of thefilter. However, materials with high coupling coefficients quitetypically have larger magnitude temperature coefficients, in comparisonwith low coupling coefficient materials. For example, 42Y-X LiTaO₃,which is considered to be a high coupling coefficient material, has acoupling coefficient of K²=4.7% and a temperature coefficient of −45ppm/° C. On the other hand, ST-Quartz, which is considered to be a lowcoupling coefficient material, has a coupling coefficient of K²=0.12%and a temperature coefficient of 0 ppm/° C. at room temperature.Generally speaking, frequency drift due to temperature variation infilters made of high coupling coefficient material is larger thanfilters made of low coupling coefficient material. Also, usually highcoupling coefficient materials have poorer Q factors than the lowcoupling coefficient materials. A SAW resonator made of 42Y-X LiTaO3 hasa Q ranging from 1000 to 2000, whereas a SAW resonator made of ST-Quartzcould have a Q of over 10,000. The difference in the Q factors ofresonators made of the different materials shows up as a difference inthe steepness of the filter transition-band.

In contrast to high coupling coefficient materials, low couplingcoefficient materials (K²<2%) are used for narrow band type band pass orband-reject filter designs. Due to the characteristics of high Q andsmall temperature coefficient mentioned above, these low couplingcoefficient material type narrow band band pass or band-reject filtersalways have steeper transition band and smaller temperature dependentfrequency drift than the high coupling coefficient material type wideband band pass or band-reject filters. Therefore, the low couplingcoefficient materials are often used for narrower band type filtersdesigns.

Table 1 below includes a list of different materials (Material Name)that can be used for fabricating SAW, BAW and FBAR devices, and furtherdefines specifically the type of devices the respective material may beused to fabricate (Filter Type). Table 1 also includes an acoustic wavevelocity (Velocity), the electro-mechanical coupling coefficient(Coupling Coefficient (K²)) and the Temperature Coefficient at RoomTemperature (Temperature Coefficient at Room Temp) for each material inthe table.

TABLE 1 Materials used in SAW, FBAR and BAW devices Temperature CouplingCoefficient at Material Velocity Coefficient Room Temp Name Filter type(m/s) (K²) (%) (ppm/° C.) ST-Quartz SAW 3150 0.12 0 LST-Quartz SAW 39480.11 0 STW-Quartz SAW 3700 to 0.17 to 0 5040 0.34 48.5Y-26.7X SAW 27350.31 1.1 Langasite SiO₂/ZnO/ SAW 10,000 1.2 0 Diamond SiO₂/AlN/ SAW11,000 0.65 0 Diamond 45X-Z Li₂B₄O₇ SAW 3440 1 0 (−15)Y-75X SAW 4120 1.61.5 Li₂B₄O₇ (−42.7)Y-90X SAW 6700 2 6.5 Li₂B₄O₇ X-112Y SAW 3288 0.64 −18LiTaO₃ Y-Z LiTaO₃ SAW 3230 0.66 −35 36 to 42Y-X SAW 4200 4.7 −45 LiTaO₃128Y-X SAW 3992 5.5 −74 LiNbO₃ Y-Z LiNbO₃ SAW 3488 4.9 −84 41Y-X LiNbO₃SAW 4792 17.2 −78 64Y-X LiNbO₃ SAW 4742 11.3 −79 ZnO FBAR 6080 8.5 −60AlN FBAR 11300 6.5 −25 ZnO BAW 6080 7.5 −48 AIN BAW 11300 6 −22

In some embodiments, a first type filter with a lower couplingcoefficient and lower temperature coefficient is fabricated with a firstmaterial that includes at least one of: Quartz; Langasite;SiO₂/ZnO/Diamond; SiO₂/AlN/Diamond; Li₂B₄O₇; AlN/Li₂B₄O₇; LiTaO₃;LiNbO₃; SiO₂/LiTaO₃; SiO₂/LiNbO₃; AlN; and combinations thereof.

In some embodiments, a second type filter with a higher couplingcoefficient is fabricated with a second material that includes at leastone of: Quartz; Langasite; SiO₂/ZnO/Diamond; SiO₂/AlN/Diamond; Li₂B₄O₇;AlN/Li₂B₄O₇; LiTaO₃; LiNbO₃; SiO₂/LiTaO₃; SiO₂/LiNbO₃; ZnO; AlN; andcombinations thereof.

There is overlap in the types of materials that can be used to make thefilters having the different material properties, i.e. high/low couplingcoefficient and high/low temperature coefficient. However, as long asthe materials that are used have respective high and low properties withrespect to one another, then a cascaded filter with desired propertiescan be designed and fabricated. Cascaded filter parameters that may beaffected by the selection of materials may include, but are not limitedto, the width of the wide band portion of the filter response, foreither of band pass or band-reject, the amount of temperature dependentfrequency drift of the response and the steepness of the transitionbands.

By using SAW, FBAR and/or BAW design technologies, some embodiments ofthe invention result in economically low cost devices having a compactphysical size. An aspect of the invention is to cascade at least twoSAW, FBAR or BAW design type filters, at least one filter fabricatedfrom a material that has a low temperature coefficient enabling lowtemperature dependent frequency drift and at least one filter fabricatedfrom a material that has a large electro-mechanical coupling coefficientenabling a wide band (>3%) pass band or wide band reject-band. Acombination of the at least two filters whose materials have thesedifferent characteristics achieves an overall filter performance withfilter parameters that include wide band pass band or wide bandreject-band, steep transition band and low temperature dependentfrequency drift of the overall cascaded frequency filter response.

In some embodiments of the invention a band reject-type filterfabricated with a material having a low temperature coefficient iscascaded, with one of a band reject-type filter or a band pass typefilter, which is fabricated with a material having a high couplingcoefficient to achieve a wide band filter with an ultra-narrowtransition band and very stable temperature dependent frequency drift ofthe cascaded filter response.

In a particular example of a cascaded filter implemented according to anembodiment described herein, the cascaded filter has a very wide passband/reject-band (>3% or >60 MHz@1.93 GHz) and a very steeptransition-band (<0.5% or <10 MHz@1.93 GHz) and a very stabletemperature dependent frequency drift (<360ppm over −40° C. to 80° C.)

FIG. 3 illustrates a block diagram of two filters, Filter A and Filter Bcascaded together. In some embodiments, Filter A is a narrow band filterfabricated using a substrate material having a low coupling coefficientas compared to the coupling coefficient of the material used tofabricate Filter B. The material used to fabricate Filter A has asmaller magnitude temperature coefficient as compared to the temperaturecoefficient of the material used to fabricate Filter B and as suchFilter A has a low temperature dependent frequency drift as compared tothe temperature dependent frequency drift of Filter B. Based on thematerial properties of the material used to fabricate Filter A, Filter Aprovides a narrow transition band as compared to the transition band ofFilter B.

Filter B is a wide band filter fabricated using a substrate materialhaving a higher coupling coefficient as compared to the couplingcoefficient of the material used to fabricate Filter A. Based on thematerial properties of Filter B, Filter B provides a wide pass band orrejection band.

The frequency response of the two cascaded filters, Filter A and FilterB, provides a wide pass band or wide rejection band with a narrowtransition band on at least one edge of the wide pass band or widerejection band. In some embodiments, when Filter A has a filter responsewith only a single narrow band rejection band, the filter response ofcascaded Filters A and B has only a narrow transition band on one sideof the wide band pass band or rejection band, either the high frequencyend or the low frequency end, depending on the design parameters ofFilter A. In some embodiments, when Filter A has a filter response withat least two narrow band rejection bands, the filter response ofcascaded Filters A and B has narrow transition bands on both sides ofthe wide band pass band or rejection band depending.

Some embodiments of the invention provide a new type of cascaded FBARtype band pass or band-reject filter with much narrower transitionbands, improved temperature stability and also with a wide pass band orrejection band as compared to a single die FBAR type band reject filteror single die FBAR type band pass filter.

Aluminum Nitride (AlN) and Zinc Oxide (ZnO) thin films are popular andwidely used piezoelectric materials for FBAR type devices. AlN has atemperature coefficient of approximately −25 ppm/° C. and a couplingcoefficient of approximately 6.5%. ZnO has a temperature coefficient ofapproximately 60 ppm/° C. and a coupling coefficient of approximately8.5%. AlN has a better Q factor in comparison with the ZnO. Using AlN,which has a larger Q factor and a better temperature stability than theZnO, as a material for fabricating a filter provides a filter with asteep transition band and a low temperature dependent frequency drift.Using ZnO, which has a higher coupling coefficient than AlN, as amaterial for fabricating a filter provides a filter with a wider bandpass band or wide band reject-band filter. By cascading two FBAR typefilters, one of which is a band reject type filter fabricated using AlNand the other one of which is either a band pass type filter or a bandreject type filter fabricated using ZnO, a new FBAR filter having filterparameters that include a narrower transition band, wider band pass bandor wider band rejection band, and lower temperature dependent frequencydrift, than either of the single die type AlN or ZnO FBAR filters couldindividually provide, is obtained. In alternative implementations, theband reject type FBAR filter made of AlN mentioned above can be replacedby either a SAW band reject filter or a BAW band reject filter that hasfilter characteristics such as steep transition band and small frequencydrift over a temperature range to obtain a new performance band passtype or band reject type filter.

In a given filter design, the filter transition band steepness, themaximum frequency drift over operating temperature range, as well asmanufacturing tolerances for a specific material, all have to be takeninto consideration for meeting guard-band specifications. In general, asingle material substrate filter fabricated with a high couplingcoefficient material having a wider band type response and a largeramount of temperature dependent frequency drift needs a much widerguard-band specification than the low coupling coefficient materialnarrow band type filters.

In some embodiments, a cascaded filter includes at least one firstfilter, each first filter being a band-reject type filter having a firstset of filter parameters that are a function of a first material used tofabricate the at least one first filter. The cascaded filter alsoincludes at least one second filter, each second filter having a secondset of filter parameters that are a function of a second material usedto fabricate the at least one second filter. Each second filter is oneof a band-reject type filter and a band pass type filter. The at leastone first filter and the at least one second filter are cascadedtogether to form the cascaded filter. In some embodiments, the firstmaterial and the second material are different materials. The cascadedfilter has a third set of filter parameters that are a function of boththe first material and the second material.

In some embodiments, the cascaded filter includes two separate dies,which are made of different materials. In some embodiments a same filterdesign technology is used for both filters, but different materials areused for the respective filters. For example, both filters may bedesigned and fabricated using SAW, FBAR or BAW. In some embodiments adifferent filter design and fabrication technology may be used tofabricate the different filters and different materials are used for therespective filters. In a first example, a first filter is fabricatedusing SAW and a second filter is fabricated using FBAR. In a secondexample, a first filter is fabricated using FBAR and a second filter isfabricated using BAW. In a third example, a first filter is fabricatedusing SAW and a second filter is fabricated using BAW. In a fourthexample, a first filter is fabricated using BAW and a second filter isfabricated using FBAR. Other combinations of filters are contemplatedbased on permutations of the different filter design and fabricationtechnologies. As the two cascaded filters could be a mixed combinationof SAW, FBAR and BAW filters, this cascaded filter design has a widevariety of design flexibilities.

Regardless of the filter design and fabrication technology used tofabricate each filter, at least one filter of a first filter type isfabricated using a low coupling coefficient material with a lowtemperature coefficient, relative to a material used to fabricate atleast one filter of a second filter type, to generate a narrow bandfilter with steep transition band and low temperature dependentfrequency drift. At least one filter of the second filter type isfabricated using a high coupling coefficient material, relative to amaterial used to fabricate at least one filter of the first filter type,to generate a wide band type filter.

Due to the different characteristics of the materials of the twofilters, the narrow band band-reject type filter can be utilized toimprove the two drawbacks that the wide band type filter has, namely anon-steep transition-band and a large temperature dependent frequencydrift over the operating temperature range. As long as the narrow bandband-reject type filter has a wide enough reject-band and deep enoughrejection level to compensate for the drift of the frequency response ofthe wide band filter over temperature variation, the frequency responseof the two filters cascaded together will provide a performance havingall beneficial features of each of the two types of filters, namely: 1)a wide pass band or reject-band; 2) a steep transition-band on at leastone edge of the wide passband or reject-band; and 3) a low temperaturedependent frequency drift of the filter response.

In some embodiments, the two filters share one package. In someembodiments, the cascaded filter package is smaller than if the twofilters were separately packaged.

The two separate dies are cascaded electrically through wires and padsinside the package. In some implementations the two cascaded filters areelectrically coupled together via short wires and/or circuit pads withinthe package. In such an implementation there is almost no additionalloss, enabling the overall cascaded filter design to achieve a desiredlow insertion loss.

In some embodiments, a link directly electrically connects the firstfilter and the second filter. For example, short wire bonds may be usedto directly connect one filter to the other. An example of a short wirebond 910 directly connecting Filter A and Filter B in package 900 isillustrated in FIG. 9A.

In some embodiments, at least one first filter and at least one secondfilter are each respectively electrically connected to a shared point ofconnection within the package. In some embodiments, short wire bonds mayconnect each filter to a circuit pad located within the package and theelectrical connection between the filters is made via the shared circuitpad. An example of a first short wire bond 915 connecting Filter A to apad 930 in package 920 and a second short wire bond 925 connectingFilter B to the pad 930 is illustrated in FIG. 9B.

In some embodiments, a flip-chip bonding technology can be used to bondeach of the filters to the package using metal bumps. An example of thedie of Filter A and the die of Filter B connected to package 940 viametal bumps 950 is illustrated in FIG. 9C. Circuit paths within thepackage between contact points of the metal bump connections can provideelectrical connection between the filters.

The above examples are merely some of the possible implementations forcascading and packaging multiple die filters. Other manners of packagingare contemplated. Furthermore, while only two filters are illustrated inthe examples of FIGS. 9A, 9B and 9C, it is to be understood that theprinciples illustrated in these figures can apply to multi-filterimplementations.

In some embodiments, the multi-filter cascade approach is used toimplement a filter design for an irregular filter specification that hasdifferent widths in the guard-bands.

In some embodiments, only two filters are cascaded together, but the twofilters may each provide multiple pass bands or rejection bands. Forexample, a first filter fabricated using a first material may providetwo narrow band rejection bands, with narrow transition bands, that arespaced apart substantially an equivalent distance of a single wide bandpass band or wide band rejection band of a second filter fabricatedusing a second material.

In some embodiments, multiple filters could be cascaded together, eachhaving a particular filter response, which collectively provide adesired overall filter response. For example, a first filter of a firstfilter type that is fabricated using a first material may provide onenarrow band rejection band with steep transition bands at a lowerfrequency end of a wide band pass band or wide band rejection band of asecond filter of a second filter type that is fabricated using a secondmaterial. A third filter of the first filter type that is fabricatedusing the first material may provide one narrow band rejection band withsteep transition bands at a higher frequency end of the wide band passband or wide band rejection band. In some embodiments, the third filtermay be fabricated using a third material that has properties that aremore similar to the first material than the second material. In such amanner it may be possible to have transition bands with differentsteepness at the high and low frequency ends of the wide band pass bandor wide band rejection band portion of the filter.

Four different embodiments of cascaded filters will now be described fordifferent types of filter response characteristics.

Example Embodiment #1

A first example embodiment in which a wide band pass band filter iscascaded with a narrow band band-reject filter having a rejection bandat the higher frequency side of the wide band filter will now bedescribed in relation to FIGS. 4A to 4E.

FIG. 4A illustrates a graphical plot of two filter responses of a wideband pass band filter, one filter response 40 showing the response at ahigh temperature (approximately 85° C.) and one filter response 41showing the response at a low temperature (approximately −40° C.). Thewide band filter is fabricated with a material having a higher couplingcoefficient than the material used to fabricate the narrow band filter.FIG. 4A is substantially the same as FIG. 1A.

FIG. 4B illustrates a graphical plot of two filter responses of a narrowband rejection band filter, one filter response 42 showing the responseat the high temperature and one filter response 43 showing the responseat the low temperature. The narrow band filter is fabricated with amaterial having a lower coupling coefficient than the material used tofabricate the wideband filter and a lower temperature coefficient thanthe material used to fabricate the wideband filter. FIG. 4B is similarto FIG. 2B, except that the band reject filter response of FIG. 4B islocated in the frequency range of 1.450 GHz to 1.480 GHz.

FIG. 4C illustrates a graphical plot of a filter response 44 of theresulting cascaded filters at the higher temperature. The frequencyrange and attenuation range of the plot are the same as FIGS. 4A and 4B.The 3 dB bandwidth of the filter response is in the range ofapproximately 0.080 GHz, from 1.370 GHz to 1.450 GHz. The transitionband on the lower frequency side of the filter response is rather large,for example, a 20 dB down transition bandwidth is approximately 0.01GHz, from 1.36 GHz to 1.37 GHz. The transition band on the higherfrequency side of the filter response is smaller in comparison to thelower frequency side, for example, a 20 dB down transition bandwidth isapproximately 0.001 GHz, from 1.450 GHz to 1.451 GHz.

FIG. 4D illustrates a graphical plot of a filter response 45 of theresulting cascaded filters at the lower temperature. The frequency rangeand attenuation range of the plot are the same as FIG. 4C. The 3 dBbandwidth of the filter response is in the range of approximately 0.075GHz, from 1.378 GHz to 1.453 GHz. The transition band on the lowerfrequency side of the filter response is rather large, for example, a 20dB down transition bandwidth is approximately 0.010 GHz, from 1.368 GHzto 1.378 GHz. The transition band on the higher frequency side of thefilter response is smaller in comparison to the lower frequency side,for example, a 20 dB down transition bandwidth is approximately 0.001GHz, from 1.453 GHz to 1.454 GHz.

FIG. 4E illustrates a graphical plot of two filter responses of theresulting cascaded filters, one filter response 46 showing the responseat the high temperature and one filter response 47 showing the responseat the low temperature. This is essentially FIGS. 4C and 4D overlaidwith one another. On the lower frequency side of the pass band filterresponses 46,47, the transition bands are substantially parallel andhave a spacing in frequency, for a given attenuation, of approximately0.010 GHz. On the higher frequency side of the pass band filterresponses 46,47, the transition bands are narrow compared to the lowerfrequency end and are substantially parallel. The high temperature andlow temperature responses 46,47 have a spacing in frequency, for a givenattenuation, of approximately 0.003 GHz. Therefore, the higher frequencyside transition band of the pass band has approximately ⅔ lesstemperature dependent frequency drift than the lower frequency sidetransition band of the pass band.

In some embodiments, when designing an appropriate transition band for afilter based on cascading two filters in a manner as described above, aparticular consideration as part of the design process is the band widthof the rejection band of the narrow band filter. As shown in FIGS. 4Aand 4B, the bandwidth of the rejection band of the narrow band filtermust be wide enough to cover a lack of rejection caused by the frequencyresponse drift of the wide band filter over the operating temperaturerange.

Care must be taken when designing filters to ensure that the entirecascaded filter performance meets the requirement at any temperaturewithin a desired operational temperature range by properly designing theband width of the rejection band of the narrow band band-reject filter.

Example Embodiment #2

A second example embodiment in which a wide band pass band filter iscascaded with a filter having two narrow band rejection bands, onerejection band at the lower frequency side of the wide band filter andone rejection band at the higher frequency side of the wide band filter,will now be described in relation to FIGS. 5A to 5C.

FIG. 5A includes a high temperature (approximately 85° C.) filterresponse 50 and a low temperature (approximately −40° C.) filterresponse 51, which are substantially the same as the two filterresponses illustrated in FIG. 4A.

FIG. 5B illustrates a graphical plot of two filter responses of a narrowband rejection band filter, one filter response 52 showing the responseat the high temperature and one filter response 53 showing the responseat the low temperature. The narrow band filter is fabricated with alower coupling coefficient material that is stable over temperaturevariation. FIG. 5B has a first rejection band that is approximately0.030 GHz, from 1.35 GHz to 1.38 GHz and a second rejection band that isapproximately 0.030 GHz, from 1.45 GHz to 1.48 GHz.

FIG. 5C illustrates a graphical plot of two filter responses of theresulting cascaded filters, one filter response 54 showing the responseat the high temperature and one filter response 55 showing the responseat the low temperature. On the lower frequency side of the filterresponses 54,55, the transition bands are substantially parallel andhave a spacing in frequency for a given attenuation of approximately0.002 GHz. On the higher frequency side of the filter responses 54,55,the transition bands are substantially parallel and have a spacing infrequency for a given attenuation of approximately 0.002 GHz. As aresult of cascading the two filters, the overall filter response has awide band pass band with a steep transition band on each side of thepass band and very low frequency drift over temperature variation.

Example Embodiment #3

A third example embodiment in which a wide band band-reject filter iscascaded with a narrow band band-reject filter having a rejection bandat the lower frequency side of the wide band filter will now bedescribed in relation to FIG. 6A to 6C.

FIG. 6A illustrates a graphical plot of two filter responses of a wideband band-reject filter, one filter response 60 showing the response ata high temperature (approximately 85° C.) and one filter response 61showing the response at a low temperature (approximately −40° C.). Thewide band filter is fabricated with a high coupling coefficientmaterial. FIG. 6A is substantially the same as FIG. 1B.

FIG. 6B illustrates a graphical plot of two filter responses of a narrowband band-reject filter, one filter response 62 showing the response atthe high temperature and one filter response 63 showing the response atthe low temperature. The narrow band filter is fabricated with a lowercoupling coefficient material that is stable over temperature variation.FIG. 6B is similar to FIG. 2B, except that the band reject filterresponse of FIG. 6B is located in the frequency range of 1.37 GHz to1.40 GHz.

FIG. 6C illustrates a graphical plot of two filter responses of the twocascaded filters, one filter response 64 showing the response at thehigh temperature and one filter response 65 showing the response at thelow temperature. On the lower frequency side of the filter responses64,65, the transition bands are substantially parallel and have aspacing in frequency for a given attenuation of approximately 0.002 GHz.On the higher frequency side of the filter responses 64,65, thetransition bands are substantially parallel and have a spacing infrequency for a given attenuation of approximately 0.010 GHz. Therefore,the lower frequency side transition band of the reject-band hasapproximately ⅘ less temperature dependent frequency drift than thehigher frequency side transition band of the reject-band.

Example Embodiment #4

A fourth example embodiment in which a wide band band-reject filter iscascaded with a filter having two narrow band rejection bands, onerejection band at the lower frequency side of the wide band filter andone rejection band at the higher frequency side of the wide band filter,will now be described in relation to FIGS. 7A to 7C.

FIG. 7A includes a high temperature (approximately 85° C.) filterresponse 70 and a low temperature (approximately −40° C.) filterresponse 71, which are substantially the same as the two filterresponses illustrated in FIG. 6A.

FIG. 7B illustrates a graphical plot of two filter responses of a narrowband band-reject filter, one filter response 72 showing the response atthe high temperature and one filter response 73 showing the response atthe low temperature. The narrow band filter is fabricated with a lowercoupling coefficient material that is stable over temperature variation.FIG. 7B is substantially the same as the two filter responsesillustrated in FIG. 6B.

FIG. 7C illustrates a graphical plot of two filter responses of theresulting cascaded filters, one filter response 74 showing the responseat the high temperature and one filter response 75 showing the responseat the low temperature. On the lower frequency side of the filterresponses 74,75, the transition bands are substantially parallel andhave a spacing in frequency for a given attenuation of approximately0.002 GHz. On the higher frequency side of the filter responses 74,75,the transition bands are substantially parallel and have a spacing infrequency for a given attenuation of approximately 0.002 GHz. As aresult of the two cascaded filters, the overall filter response has awide band rejection band with a steep transition band on each side ofthe reject-band and very low frequency drift over temperature variation.

The filter response parameters associated with the examples of FIGS.4A-4E, 5A-5C, 6A-6C and 7A-7C are merely exemplary in nature. Theparameters involved in designing filters for any given application, suchas, but not limited to, transition band steepness, pass band orband-reject bandwidth and temperature dependent frequency drift, areimplementation specific.

In some embodiments, matching networks may be used in conjunction withthe cascaded filter design. In some embodiments the use of matchingnetworks may improve performance of the overall filter. FIG. 8 is ablock diagram illustrating how matching circuits may be used withrespect to a cascaded filter design. In FIG. 8, Filter A and Filter Bare cascaded together in package 860 and provide an overall filterresponse. A first matching circuit 810 is coupled to an input of FilterA. The first matching circuit 810 has a pair of inputs 812,814. An inputto the cascaded filter may be applied at inputs 812,814. A secondmatching circuit 820 is coupled to an output of Filter B. The secondmatching circuit 820 has a pair of outputs 822,824. An output of thecascaded filter is provided at outputs 822,824. A third matching circuit830 is coupled to Filter A for matching Filter A itself. The thirdmatching circuit 830 is grounded. A fourth matching circuit 840 iscoupled to Filter B for matching Filter B itself. The fourth matchingcircuit 840 is grounded. A fifth matching circuit 850 is coupled to aconnection between Filter A and Filter B for matching the connectionbetween Filter A and filter B. The fifth matching circuit 850 isgrounded.

FIG. 8 illustrates a set of matching networks for a filter comprisingtwo cascaded filters of the type generally described above. In someembodiments, not all of the matching networks described in FIG. 8 areused in conjunction with the cascaded filters. The number of matchingnetworks and where they are located with respect to the cascaded filterare implementation specific.

Matching networks can be realized by using discrete components ortransmission lines or some combination thereof. In some implementationthe matching networks may include discrete inductors.

A method of fabricating a filter will now be described with reference toFIG. 10.

A first step 10-1 of the method involves cascading at least one firstfilter with at least one second filter. Each first filter is aband-reject type filter having a first set of filter parameters that area function of a first material used to fabricate the at least one firstfilter. Each second filter has a second set of filter parameters thatare a function of a second material used to fabricate the at least onesecond filter. In addition, each second filter is one of a band-rejecttype filter and a band pass type filter. The first material and thesecond material are different materials. The filter has a third set offilter parameters that are a function of both the first material and thesecond material.

While the method above describes fabricating a filter using two filters,it is to be understood that, in accordance with embodiments of theinvention described above, more than a single filter of each first andsecond filter may be used in cascading the filters such that there maybe more than one first filter and more than one second filter.Furthermore, if more than one first filter is used, the materials usedfor the more than one first filter may not necessarily be the exactsame, but are more similar to each other than to the material used forthe more than one second filter. The same applies for more than onesecond filter.

A method of filtering a filter will now be described with reference toFIG. 11.

A first step 11-1 of the method involves providing a signal to an inputof a first filter. The first filter is a band-reject type filter havinga first set of filter parameters that are a function of a first materialused to fabricate the at least one first filter.

A second step 11-2 of the method involves filtering the signal using thefirst filter thereby producing an output of the first filter.

A third step 11-3 of the method involves providing the output of thefirst filter to a second filter, the second filter having a second setof filter parameters that are a function of a second material used tofabricate the at least one second filter, each second filter being oneof a band-reject type filter and a band pass type filter.

A fourth step 11-4 of the method involves filtering the output of thefirst filter using the second filter thereby producing an output of thesecond filter.

The first material and the second material are different materials. Thecombination of the at least one first filter and the at least one secondfilter used to filter the signal has a third set of filter parametersthat are a function of both the first material and the second material.

While the method above describes filtering a signal using two filters,it is to be understood that, in accordance with embodiments of theinvention described above, more than a single filter of each first andsecond filter may be used to filter a signal such that there may be morethan one first filter and more than one second filter. Furthermore, ifmore than one first filter is used, the materials used for the more thanone first filter may not necessarily be the exact same, but are moresimilar to each other than to the material used for the more than onesecond filter. The same applies for more than one second filter.

Some embodiments of the invention also may provide a low cost andcompact size type duplexer or multiplexer with the performance of lowinsertion loss, high power handling capability, wide band pass band orwide band rejection band, narrow transition band and low temperaturedependent frequency drift by applying aspects of the present cascadedfilter invention.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practised otherwise than as specifically described herein.

1. A filter comprising: at least one first filter having a first set offilter parameters that are a function of a first material used toimplement the at least one first filter; at least one second filterhaving a second set of filter parameters that are a function of a secondmaterial used to implement the at least one second filter; wherein theat least one first filter is coupled to the at least one second filterto form a composite filter having a set of composite filter parameters,the composite filter parameters comprising a temperature dependentfrequency drift that is primarily governed by a temperature coefficientof one of the first material and the second material.
 2. The filter ofclaim 1, wherein the composite filter parameters comprise a filterbandwidth that is primarily governed by a mechanical couplingcoefficient of the other of the first material and the second material.3. The filter of claim 1, wherein the at least one first filter iscoupled in series with the at least one second filter.
 4. The filter ofclaim 1, wherein the at least one first filter is a band reject filterhaving at least one rejection band, the at least one rejection bandhaving transition bands that are steeper than transition bands of the atleast one second filter.
 5. The filter of claim 3, wherein the at leastone second filter is a band pass filter having a pass band wider thanthe rejection band of the at least one first filter.
 6. The filter ofclaim 3, wherein the at least one second filter is a band reject filterhaving a rejection band wider than the rejection band of the at leastone first filter.
 7. The filter of claim 1, wherein the at least onefirst filter is any one of a surface acoustic wave (SAW) filter, a thinfilm bulk acoustic resonator (FBAR) filter and a bulk acoustic wave(BAW) filter.
 8. The filter of claim 1, wherein the at least one secondfilter is any one of a surface acoustic wave (SAW) filter, a thin filmbulk acoustic resonator (FBAR) filter and a bulk acoustic wave (BAW)filter.
 9. The filter of claim 1, wherein the at least one first filterand the at least one second filter are mounted in a common package. 10.The filter of claim 1, further comprising at least one circuit matchingelement.
 11. A method of making a filter, comprising: fabricating atleast one first filter using a first material, the at least one firstfilter having a first set of filter parameters that are a function ofthe first material; fabricating at least one second filter using asecond material, the at least one second filter having a second set offilter parameters that are a function of the second material; couplingthe at least one first filter to the at least one second filter to forma composite filter having a set of composite filter parameters, thecomposite filter parameters comprising a temperature dependent frequencydrift that is primarily governed by a temperature coefficient of one ofthe first material and the second material.
 12. The method of claim 11,wherein the composite filter parameters comprise a filter bandwidth thatis primarily governed by a mechanical coupling coefficient of the otherof the first material and the second material.
 13. The method of claim11, wherein coupling the at least one first filter to the at least onesecond filter comprises coupling the at least one first filter in serieswith the at least one second filter.
 14. The method of claim 11, whereinfabricating at least one first filter comprises fabricating the at leastone first filter as a band reject filter having at least one rejectionband, the at least one rejection band having transition bands that aresteeper than transition bands of the at least one second filter.
 15. Themethod of claim 13, wherein fabricating at least one second filtercomprises fabricating the at least one second filter as a band passfilter having a pass band wider than the rejection band of the at leastone first filter.
 16. The method of claim 13, wherein fabricating atleast one second filter comprises fabricating the at least one secondfilter as a band reject filter having a rejection band wider than therejection band of the at least one first filter.
 17. The method of claim11, wherein fabricating at least one first filter comprises fabricatingthe at least one first filter as any one of a surface acoustic wave(SAW) filter, a thin film bulk acoustic resonator (FBAR) filter and abulk acoustic wave (BAW) filter.
 18. The method of claim 11, whereinfabricating at least one second filter comprises fabricating the atleast one second filter as any one of a surface acoustic wave (SAW)filter, a thin film bulk acoustic resonator (FBAR) filter and a bulkacoustic wave (BAW) filter.
 19. The method of claim 11, comprisingmounting the at least one first filter and the at least one secondfilter in a common package.
 20. The method of claim 11, furthercomprising coupling at least one circuit matching element to at leastone of the at least one first filter and the at least one second filter.